Process of calcining gypsum



y 9 1934. 41.. c. KARRECK El Al. 1,967,029

PROCESS OF CALCINING GYPSUM Filed June 10, 1930 2 Sheets-Sheet 1 Jufly 17, 1934., L. c. KARRHCK EH AL A PROCESS OF CALCINING GYPSUM 2 Sheets- Sheet 2 FiledJune 1.930

- u I l l l l I l ll I f l I A I Y It INVE TORS ai 23 v AHORNEY Patented July 17, 1934 PROCESS or CALCINING cvrsmu Lewis G. Karrlck and William L. Ellerbeck, Salt Lake City, Utah Application June 10, 1930, Serial No. 460,133

4 Claims.

This invention relates to a new form of calcined plaster and to a process for producing it. One suitable form of apparatus for producing the plaster is herein illustrated as embodied in an 6 electrically heated kiln.

Accurate control of the temperature is vital to the production of a uniform and satisfactory calcined gypsum. According to the usual practice an externally fired kettle is used and an attempt is made to keep the temperature of the ground raw gypsum within certain rather narrow limits. During the application of heat the gypsum is continually stirred by paddles and chains fastened to a rotating vertical shaft. This process of agitation is uncertain and consumes a large amount 'of power. In the hands of skilled operators this apparatus can be made to yield a fairly uniform material, but the paddle method of agitation is unscientific and uncertain and some of the gypsum in the upper section of the kettle may be under-calcined and some held unduly near or on the over heated kettle bottom is often over-calcined, so that the properties of the production are inferior to those of the product of our invention.

Moreover some of the calcium carbonate, nearly always present in commercial gypsum, is part- 1y calcined to quick lime, and the presence of caustic lime is often objectionable. Besides this some undesirable calcium sulphide is often formed either by reaction of the gypsum with organic matter present or with carbonaceous ma-.

terial filtering in through leaks in fiues or cracks in the kettle bottom. Sometimes undesirable sulphurous acid is released owing to local reactions within the mass.

According to the present invention these and other difiiculties and objections are overcome. The gypsum mass is continuously and evenly agitated and circulated, all particles of the mass being heated identically.

It has been found advantageous to heat the gypsum in a closed kiln provided 'with means for intermittently releasing the steam given off, thus suddenly releasing the pressure. The product thus produced by continuously disrupting the crystals by sudden release of pressure has a materially greater cementing value and sand-carrying capacity than the kettle-made product. Other features and advantages will hereinafter appear.

In the accompanying drawings Figure 1 is a sectional longitudinal view through a. kiln and its'mountings. Figure 2 is a similar transverse 55 view of the same. Figure 3 is a detail view of gait of a cam ring for opening and closing the The kiln in the form shown consists of a metal lined barrel 10 mounted on trunnions 11 and 12. To charge it it is'tumed until a central opening 13 on its periphery stands uppermost so that ground raw gypsum may be delivered into the interior of the barrel 10. Usually the kiln is charged with enough raw gypsum to more than half fill it. Then a cover 14 is caused to close over the opening, the kiln is started revolving and the heat turned on.

For heating the gypsum the kiln is provided with internal heating means, herein illustrated as electrical resistance members 15. The electrical resistance members may be in the form of rods or pipes 16 extending longitudinally of the kiln provided with covered electric heating resistance windings 17 connected to commutator or brush or other suitable connections such as revolving mercury contacts 18. The heat, in this form of the invention, is derived from an electric current provided by mains 19.

In addition to the opening 13 there may be a second opposite normally closed opening 20, from 3 which the calcined gypsum or plaster is discharged into a suitable hopper 21, so that it may be stored or packed.

After the kiln has received a charge of raw ground gypsum and the opening 13, is closed, 95 thus closing the kiln, the heating is begun. The heating devices are regulated to maintain the mass for the necessary time within the theoretically critical temperatures for driving off the -desired amount of the water. The revolving of the kiln barrel circulates the powdered gypsum around the heating devices and uniformly mixes the gypsum so that the heat is uniformly distributed with the result that the highest quality product may be obtained even by ordinary labor.

In order to control the heating and to vent the steam released from the hot gypsum a valve is provided for the kiln. In the form here shown the valve is normally closed and is automatically released every time the rotation of the kiln loo brings it to its most elevated position. For this purpose there is provided a fixed cam ring 22, which is shown as about midway of the length of the kiln, adjacent the openings 13 and 20 which may also serve as the valves.

For this purpose each opening has a cover 14 pivoted in a lug 24 on the body of the kiln. Each cover has a rabbeted edge 25 fitting the opening closely and is held down on the opening flange 26 by a cam follower 27 running on no the inside of the ring 22. The followers are on the ends of strong leaf springs 28, pivoted intermediate their ends on ears 29 on the covers, and embrace adjusting screws on which turn adjustable screws or nuts 31, to vary the tension by which each follower 27 holds its cover 14 against the kiln.

To enable the covers 14 to serve as relief valves, the cam 22 is provided with a movable section at its uppermost'point. The section 32 normally occupies the position A of Figure 3 with the result that every time a follower 27 reaches the uppermost point in the revolution of the kiln it releases its spring 28 and permits escape of the steam generated by heating the gypsum. This mechanically varies the vapor pressure of the material being heated.

The further turning of the kiln causes the follower to ride down the section 32 to the internal surface of the cam 22. Any desired number of openings 13 may be provided, and when each is closed by the section 32, its spring allows its cover 14 to adjust itself on any gypsum which may settle on the flange 26.

When it is desired to charge the kiln the section 32 is swung by its control arm 33, a connecting rod 34, and bell crank 35 to the full line position of Figure 3, so that the on-coming follower 27 rides on to the outside of the ring 22, thus holding its cover 14 open, enabling it to be opened wide for charging the kiln by a hopper 36 provided with a slide-valve gate 37.

When the kiln is charged the follower 27 is brought under the section 32, the section 32 is then set at the dotted line B position, and the kiln turned so the follower 27 rides down under the ring 22 closing the kiln. The kiln then r0- tates forward, and is heated by the coils 17. The ground gypsum is continuously turned over by the rotation and is more thoroughly mixed by suitable bafiies 38, 39, which spring from the ends of the kiln and incline toward the center so that the gypsum slides down the baffles. Preferably there is a pair of baffles 39 and 40 which extend part way from the periphery to the center and terminate practically at the edge of one or both openings 13 and 20 in order that the kiln in rotating will readily discharge its contents through the openings 13 and 20. Other similar peripheral baflies 38 (only one being shown) preferably extend from the ends beyond the middle of the kiln so the swirling gypsum tends to be carried from each end more than half way to the other end thus ensuring perfeet mixing during calcination.

For emptying the kiln the section 32 is held in its depressed (full line) position for one revolution then returned to original position while the kiln is revolving, which permits the kiln to empty. This is a normally dusty operation even though the falling gypsum falls into a hopper 21 fitting the periphery of the kiln midway as closely as the mechanism allows. The kiln and its mechanism are therefore shown as enclosed in a housing having sides 42 and a. top formed by the operators floor 43. This keeps the dust confined and conserves heat.

In order to keep any uncalcined gypsum out of the bin 44 into which the hopper and chute 21 delivers the calcined gypsum, the connecting chute 21 is mounted in a slanting floor 45, e tending entirely from one wall 42 to the other. The floor 45 carries the dust down to a conveyor 46 which carries it off for further calcining or other disposal, as desired.

The hopper and chute 21 is formed with wall 47 hinged on its lower side with the floor 45 so that the wall 47 may normally lie as part of the floor and close the hopper and chute 21 from communicating with bin 44. When the kiln is to be cleared the wall 47 is swung up around its hinge 48, opening the hopper and chute 27 so the calcined ypsum is directed into the bin 44. During the operation the calcined gypsum is prevented from running into space occupied by the conveyor 46.

Here the hot gypsum comes in contact with coils of pipes diagrammatically shown as 49 through which powdered gypsum, air or water flows. These coils absorb heat at a regulated rate and the contained air or water if used, may be used in preheating the gypsum going to the hopper 36 or for other useful purposes.

The bottom of the bin 44 has a slanting floor 50 down which the cooled calcined gypsum flows to a conveyor 51 which carries it to storage or to packing machinery.

The side or door 4'7 may be operated by a rod 52 and a link 53 connected to a handle 54 on the floor 43 like the handle 33.

The kiln 10 is preferably covered with a heat insulating layer to conserve. heat, and the electrical connections to the heaters 1'7 may lie within this layer.

It is found that a given gypsum calcined under the foregoing conditions is capable of carrying from ten per cent to over twenty per cent more sand in a mortar mix of a determined consistence or plasticity than when the gypsum is calcined by the kettle method. Moreover it is found that calcined gypsum thus produced required upward of five per cent more of water than does kettle calcined gypsum when subjected to the usual laboratory slumping test. Under the microscope the crystals of the gypsum calcined according to the procedure just described are symmetrical with rounded edges instead of irregular with jagged edges which are in part exfoliations of exploded gypsum particles.

Owing to the close control of calcination attained by this invention no local overheating of any part of the charge is possible and it will be maintained well below the temperature at which any carbonate of lime begins to give up any carbon dioxide. The kiln therefore provides means for producing gypsum products free from deleterious lime or calcium sulphide or sulphur dioxide. Moreover after or during the time that the gypsum is being calcined, accelerators, retarders, fibres, colors and other ingredients may be added to the kiln contents and the kiln then used as a mixer to obtain a uniformly mixed product. Moreover the bin 44 as used provides for cooling under regulated conditions.

One or both trunnions may be made hollow and the opening continued into the axis to provide for measuring the internal temperature by a thermometer or thermocouple 58.

Having thus described certain embodiments of the invention, what is claimed is:

1. The process of calcining ground gypsum which consists in tumbling it over heating members so that steam is produced and releasing the steam pressure intermittently.

2. The process of calcining pulverized solid material containing chemically combined water to remove vaporizable materials therefrom which comprises heating said solid materials in a turbulent state with internal heating means, confining said material and vaporizable product under inmemes creasing pressure, suddenly releasing the pres sure at intervals to permit sudden expulsion of vapors from within the particles of materials treated.

3. The process of calcining pulverized solid materiai to remove volatilizable materials therefrom which comprises heating said solid materials in a turbulent state with internal heating means, confining said material and volatiles produced under increasing pressure, suddenly releasins the pressure to permit sudden expulsion of volatiles from within the particles of materials treated, repeating the step of building up and sudden releases of pressure until the desired 

